Referring to FIG. 1, a color CRT 10 generally comprises an evacuated glass envelope consisting of a panel 12, a funnel 13 sealed to the panel 12 and a tubular neck 14 connected by the funnel 13, an electron gun 11 centrally mounted within the neck 14, and a shadow mask 16 removably mounted to an inner sidewall of the panel 12. A three color phosphor screen is formed on the inner surface of a display window or faceplate 18 of the panel 12.
The electron gun 11 generates three electron beams 19a or 19b, said beams being directed along convergent paths through the shadow mask 16 to the screen 20 by means of several lenses of the gun and a high positive voltage applied through an anode button 15 and being deflected by a deflection yoke 17 so as to scan over the screen 20 through-apertures or slits 16a formed in the shadow mask 16.
In the color CRT 10, the phosphor screen 20, which is formed on the inner surface of the faceplate 18, comprises an array of three phosphor elements R, G and B of three different emission colors arranged in a cyclic order of a predetermined structure of multiple-stripe or multiple-dot shape and a matrix of light-absorptive material 21 surrounding the phosphor elements R, G and B, as shown in FIG. 2.
A thin film of aluminum 22 or electro-conductive layer, overlying the screen 20 in order to provide a means for applying the uniform potential applied through the anode button 15 to the screen 20, increases the brightness of the phosphor screen, prevents ions from damaging the phosphor screen and prevents the potential of the phosphor screen from decreasing. And also, a resin film 22' such as lacquer is applied to the phosphor screen 20 before forming the aluminum thin film 22, so as to enhance the flatness and reflectivity of the aluminum thin film 22.
In a photolithographic wet process, which is well known as a prior art process for forming the phosphor screen, a slurry of a photosensitive binder and phosphor particles is coated on the inner surface of the faceplate. It does not meet the higher resolution demands and requires a lot of complicated processing steps and a lot of manufacturing equipments with the use of a large quantity of clean water, thereby necessitating high cost in manufacturing the phosphor screen. In addition, it discharges a large quantity of effluent such as waste water, phosphor elements, 6th chrome sensitizer, etc.
To solve or alleviate the above problems, an improved process of electro-photographically manufacturing the screen utilizing dry-powdered phosphor particles is developed.
U.S. Pat. No. 4,921,767, issued to Datta at al. on May 1, 1990, discloses the improved method of electro-photographically manufacturing the phosphor screen assembly using dry-powdered phosphor particles through a series of steps represented in FIGS. 3A to 3E, as is briefly explained in the following.
After the panel 12 is washed, an electro-conductive layer 32 is coated on the inner surface of the faceplate 18 of the panel 12 and the photo-conductive layer 34 is coated thereon, as shown in FIG. 3A. Conventionally, the electro-conductive layer 32 is made from an inorganic conductive material such as tin oxide or indium oxide, or their mixture, and preferably, from a volatilizable organic conductive material such as a polyelectrolyte commercially known as polybrene(1,5-dimethyl-1,5-diaza-undecamethylene polymethobromide, hexadimethrine bromide), available from Aldrich Chemical Co.
The polybrene is applied to the inner surface of the faceplate 18 in an aqueous solution containing about 10 percent by weight of propanol and about 10 percent by weight of a water-soluble adhesion-promoting polymer (poly vinyl alcohol, polyacrylic acid, polyamide and the like), and the coated solution is dried to form the conductive layer 32 having a thickness from about 1 to 2 microns and a surface resistivity of less than about 10.sup.8 .OMEGA./.quadrature. (ohms per square unit).
The photo-conductive layer 34 is formed by coating the conductive layer 32 with a photo-conductive solution comprising a volatilizable organic polymeric material, a suitable photo-conductive dye and a solvent. The polymeric material is an organic polymer such as polyvinyl carbazole, or an organic monomer such as n-ethyl carbazole, n-vinyl carbazole or tetraphenylbutatriene dissolved in a polymeric binder such as polymethylmethacrylate or polypropylene carbonate. The photo-conductive composition contains from about 0.1 to 0.4 percent by weight such dyes as crystal violet, chloridine blue, rhodamine EG and the like, which are sensitive to the visible rays, preferably rays having wavelength of from about 400 to 700 nm. The solvent for the photo-conductive composition is an organic matter such as chlorobenzene or cyclopentanone and the like which will produce as little contamination as possible on the conductive layer 32. The photo-conductive layer 34 is formed to have a thickness from about 2 to 6 microns.
FIG. 3B schematically illustrates a charging step, wherein the photo-conductive layer 34 overlying the electro-conductive layer 32 is positively charged in a dark environment by a conventional positive corona discharger 36. As shown, the charger or charging electrode of the discharger 36 is positively applied with direct current while the negative electrode of the discharger 36 is connected to the electro-conductive layer 32 and grounded. The charging electrode of the discharger 36 travels across the layer 34 and charges it with a positive voltage in the range from +200 to +700 volt.
FIG. 3C schematically shows an exposure step, wherein the charged photo-conductive layer 34 is exposed through a shadow mask 16 by a xenon flash lamp 35 having a lens system 35' in the dark environment. In this step, the shadow mask 16 is installed on the panel 12 and the electro-conductive layer 32 is grounded. When the xenon flash lamp 35 is switched on to shed light on the charged photo-conductive layer 34 through the lens system 35' and the shadow mask 16, portions of the photo-conductive layer 34 corresponding to apertures or slits 16a of the shadow mask 16 are exposed to the light. Then, the positive charges of the exposed areas are discharged through the grounded conductive layer 32 and the charges of the unexposed areas remain in the photo-conductive layer 34, thus establishing a latent charge image in a predetermined array structure, as shown in FIG. 3C. In order to exactly attach light-absorptive materials, it is preferred that the xenon flash lamp 35 travels along three positions while coinciding with three different incident angles of the three electron beams.
FIG. 3D schematically shows a developing step which utilizes a developing container 35" containing dry-powdered light-absorptive or phosphor particles and carrier beads for producing static electricity by coming into contact with the dry-powdered particles. Preferably, the carrier beads are so mixed as to charge the light-absorptive particles with negative electric charges and the phosphor powders with positive electric charges when they come into contact with the dry-powdered particles.
In this step, the panel 12, from which the shadow mask 16 is removed, is put on the developing container 35" containing the dry-powdered particles, so that the photo-conductive layer 34 can come into contact with the dry-powdered particles. In this case, the negatively charged light-absorptive particles are attached to the positively charged unexposed areas of the photo-conductive layer 34 by electric attraction, while the positively charged phosphor particles are repulsed by the positively charged unexposed areas but attached by reversal developing to the exposed areas of the photo-conductive layer 34 from which the positive electric charges are discharged.
FIG. 3E schematically represents a fixing-step by means of infrared radiation. In this step, the light-absorptive and phosphor particles attached in the above developing step are fixed together and onto the photo-conductive layer 34. Therefore, the dry-powdered particles includes proper polymer components which may be melted by heat and have proper adhesion.
The steps of charging, exposing, developing and fixing are repeated for the three different phosphor particles. Moreover, the same process of the above steps can be repeated also for the black matrix particles before or after the three different phosphor particles are formed.
After the three different phosphor particles and the black matrix particles are formed through the above process, a lacquer film is formed through a lacquering step and an aluminum thin film is formed through an aluminizing step respectively by a conventional method. Thereafter, the faceplate panel 12 is baked in air at a temperature of 425.degree. C., for about 30 minutes to drive off the volatilizable constituents such as the organic solvents from the conductive layer 32, the photo-conductive layer 34, the phosphor elements and the lacquer film, thereby forming a screen array 20 of light-absorptive material 21 and three phosphor elements R, G and B in FIG. 2.
The conventional method of electro-photographically manufacturing the phosphor screen assembly using dry-powdered phosphor particles as described above has one problem that it requires dark environment during all the steps until the fixing step after the photo-conductive layer is formed, because the photo-conductive layer is sensitive to the visual light. Also, the fixing step of FIG. 3E is still necessary even after the developing step.
To overcome this problem, the applicant proposed a method of forming the photo-conductive layer using a photo-conductive solution responsive to the ultraviolet rays.
The solution for the photo-conductive layer 34 responsive to the ultraviolet rays, for example, may contain: an electron donor material, such as about 0.01 to 1 percent by weight of bis-1,4-dimethyl phenyl(-1,4-diphenyl(butatriene)) or 2 to 5 percent by weight of tetraphenyl ethylene (TPE); an electron acceptor material, such as about 0.01 to 1 percent by weight of at least one of trinitro-fluorenone (TNF) and ethyl anthraquinone (EAQ); a polymeric binder, such as 1 to 30 percent by weight polystyrene; and a solvent such as the remaining percent by weight of toluene or xylene.
As the polymeric binder, poly(.alpha.-methylstyrene) (P.alpha.MS), polymethylmethacrylate (PMMA), and polystyrene-oxazoline copolymer (PS-OX) may be employed instead of the polystyrene.
However, since the aforementioned 2 to 5 percent by weight of tetraphenyl ethylene (TPE) as an electron donor material has a high recrystallization speed and a large aging effect, it can not be used after 24 hours passed. The reason of the high recrystallization speed and a large aging effect are assumed that the TPE has a plane molecular structure, so that it is laminated to form the photo-conductive layer 34 mainly when applied while it coheres after having been dissolved.
The present invention has been made to overcome the above described problems, and thereby it is an object of the present invention to provide a solution for making a photo-conductive layer in dry-electrophotographically manufacturing a screen of a CRT and a method for dry-electrophotographically manufacturing the screen using the solution, which can improve the photo-conductivity of the photo-conductive layer to save energy and at the same time increase the developing density of the powdered phosphor particles and reduce the aging effect, so that the photo-conductive layer can maintain a superior photo-conductivity even after it has been stored for long time.